This invention relates to a process for the preparation of pure tertiary olefins starting from their corresponding tertiary ethers.
It is known that by reacting a low-molecular weight alcohol with a mixture of olefins the tertiary olefins only enter reaction to form alkyl-tert.alkyl-ethers, inasmuch as the other olefins either react very slowly or remain completely unaffected.
Tertiary olefins are very important starting materials for the preparation of polymers and chemicals, so that it is of the utmost importance to succeed in isolating them in the purest possible form.
Methods for the obtention of tertiary olefins are known. For example, a few methods are based on the use of H.sub.2 SO.sub.4 ; but, in addition to the corrosion and pollution problems, in methods where the acid is used there are a number of other shortcomings, among which the requirement of concentrating the acid prior to recycling is one of the most important. Other methods are based on the decomposition of the corresponding methyl ethers in the presences of suitable catalytic systems.
However, the use of the catalysts suggested heretofore for carrying out such a reaction is conducive, in the majority of the cases, to the formation of dialkyl ethers as a consequence of the dehydration of the corresponding primary alcohols.
While such a reaction is faster, the higher the working temperature a few if the conventional catalysts require the use of comparatively high temperatures, a fact which leads to the loss of alcohol and the attendant necessity of feeding fresh alcohol to the initial etherification reaction.
Moreover, the formation of dialkyl ethers necessitates more intricate installations since a separation of the dialkyl ethers from the tertiary olefin is imperative. Furthermore, the formation of a considerable amount of dialkyl ether necessitates also the dehydration of the primary alcohol prior to recycling the same, otherwise the admixture of phases is experienced in the etherification reaction and the formation of tertiary alcohols might take place.
Another drawback which is experienced when the reaction is carried out beyond certain temperature levels is the occurrence of dimerization and trimerization of the tertiary olefin which is recovered from the decomposition of the ethers.
A few of the mentioned defects can be offset when the modification reaction of the tert.alkyl ethers is carried out in the presence of a catalytic system composed of an active alumina which has been modified by partially substituting silanol groups for the superficial --OH groups, according to the teaching of our Italian Pat. No. 1,001,614 of Apr. 4, 1976. Nevertheless, the modified active alumina made according to the teaching of the aforementioned patent gives rise, when a strong temperature increases in the reaction temperature is experienced, to the formation of dialkyl ether, the result being a poorer recovery of the primary alcohol to be recycled.
It has been found that the shortcomings of the conventional art can be overcome and, most important, that a recovery of methanol of over 90% can be achieved irrespective of the working temperatures which can even exceed 400.degree. C., and which also occurs in a complete absence of any secondary reactions.
It has also been found that it is possible to prepare the tertiary olefins in a state of purity and in a high yield, starting from the tert. Alkyl ethers mentioned above, by contacting the ether concerned with a particular catalyst system. The ether is thus decomposed into the olefin and the corresponding low molecular weight alcohol and the latter in recycled and again reacted with a mixture of olefins.
In accordance with the present invention there is provided a process for the preparation of tertiary olefins starting from the corresponding tert.alkyl ethers, more particularly isobutene from methyl tert.butyl ether, said process being characterized in that a catalyst is used, which is selected from among a crystalline silica which has been modified or unmodified with oxides of metal cations and which corresponds a high specific surface area and corresponding to the following general formula: 0-1M.sub.n O.sub.m. 1SiO.sub.2, more particularly 0.0001-1M.sub.n O.sub.m. 1SiO.sub.2, wherein M.sub.n O.sub.m is the oxide of a metallic cation capable of entering the silica lattice as a substituent for silicon or as a salt of polysilicic acids, and/or a silica which has been modified with alumina and corresponds to the following general formula: EQU 0.0006-0.0025Al.sub.2 O.sub.3. 1SiO.sub.2
wherein the quantity of alumina which has been introduced enables the catalytic activity to be monitored.